The invention relates to radio communication systems. More particularly, operation of multiple radio systems in similar radio spectrums and/or located geographically near to each other.
When a few decades ago, spectrum regulations were changed to allow commercial radio applications in unlicensed bands, interest was marginal. But this interest has changed radically in the last few years. After the worldwide success of mobile telephony in licensed bands, capacity limitations and huge license fees have spurred the interest of radio applications in the unlicensed band. In the past few years, communications systems such as those operating in accordance with the Wireless Local Area Network (WLAN) IEEE 802.11 standards and the Bluetooth® standards have been increasingly deployed in the 2.4 GHz band. Moreover, new communications systems are being worked on, such as ZigBee and those resulting from the Wireless Personal Area Network (WPAN) activity under IEEE 802.15.
Radio spectrum, even unlicensed, is limited. Despite this, ubiquitous communications using several different standards is foreseen in the near future. Coexistence is not trivial as different standards follow different protocols. Moreover, regulations, initially intended to provide fair sharing, are constantly changing to allow for higher data rates, yet moving away from robustness requirements. The use of an unlicensed band poses the challenge of coexistence. In the design phase of a new communication system that has to operate in the unlicensed band, the developer has to design units that will be expected to share the band with:                Incumbent non-communications: Power unintentionally radiated by equipment, for example microwave ovens, will be a source of disturbance.        Incumbent communications: Intended radiation by other communication systems like for example WLAN, Bluetooth®, or Radio Frequency-Identification (RF-ID) will also be experienced as disturbance when no coordination is applied.        Future systems: Systems that do not exist yet but which will be built in the future can cause severe disturbances. The only known factors are the restrictions imposed upon these systems by the regulations. However, as discussed before, regulations are changing over time, making predictions rather unreliable.        
In general, the performance (say the packet-error-rate PER) of a communication system is determined by seven variablesPER=H(Pu, Bu, Ru, du, Pj, Bj, dj)where H is a monotonically increasing function for each of the parameters except Pu; for the parameter Pu, H is a monotonically decreasing function. In this formula, Pu, Pj are the transmit power levels of the intended and jamming transmitters, respectively; Bu, Bj are the respective intended and jamming transmission bandwidths; and du, dj are the respective intended and jamming duty cycles. Ru is the user information rate. Furthermore, the quality of the reception is a function of environmental factors that determine the final received signal strength of both the intended and jamming signals. Of great importance are the distances between intended transmitter and receiver, and between the jamming source and receiver. In addition other effects impact the propagation of the signals such as obstructions (shadowing), reflections, refraction, and wave guide effects to name a few. If both the intended and jamming transmitters send simultaneously on the same frequencies, the performance will depend on the Signal-to-Interference ratio (S/I) between intended and interfering power experienced at the receiver. Larger power levels, Pu, of the intended transmitter give greater protection against interference, as long as the distance between intended transmitter and receiver are not too large. Vice versa, shorter distances guarantee a larger received signal that can overcome interference. The current trends of designing more sensitive receivers in order either to increase the range or to reduce the required transmit power, do not have a positive effect on the robustness of the system.
The ratio Bu/Ru is a measure of the processing gain or coding gain of the system: a lower information rate allows for more overhead in order to combat interference either by signal spreading or by coding. The wider the bandwidths Bu, Bj, the higher the probability of overlap in the frequency domain. Increasing bandwidths also decreases the possibility of avoiding each other's transmissions in frequency. The larger the duty cycles du, dj, the larger the probability of overlap in the time domain. Increasing the duty cycles also decreases the possibility of avoiding each other's transmissions in time.
Interference mitigation by applying direct-sequence spreading or forward-error-correction coding can be useful, but is usually insufficient due to the near-far problem. That is, in ad-hoc scenarios in particular, a jamming transmitter can come very close to a receiver. The power levels received can thus be sufficiently strong to bring the front-end of the receiver into saturation, which causes clipping. As a result of the clipping (which imposes non-linear effects) the effective gain decreases (desensitization) and intermodulation products arise.
Avoidance is another method of mitigating interference. Avoidance in time can be applied by listening-before-talk or Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) as applied in IEEE 802.11 and other standards. However, this renders suboptimal solutions because the collision measurements render absolute power levels whereas the Packet Error Rate (PER) depends on the Carrier-to-Interference (C/I) ratio.
Avoidance in frequency is provided by frequency agile techniques such as Dynamic Frequency Selection (DFS). In this method, the system measures where in the frequency band other transmitters are active, and subsequently avoids these frequency segments. This is fine when potential jammers broadcast their presence continuously, for example on a control channel. However, measuring on bursty data channels results in unreliable measurements. Hopping provides better mitigation methods based on frequency avoidance. Because of the large isolation between the intended signal and the jammer when the hopper and jammer do not coincide, rather good robustness can be obtained. However, frequency hopping only works when the jammers are narrowband; likewise, time hopping only works when jammers have a low duty cycle. Incumbent systems in the unlicensed bands usually are bandwidth restricted but are rarely duty cycle restricted, posing a problem for time hopping systems like Ultra-Wideband (UWB) Impulse Radio.
As the usage of the unlicensed bands is intensified, coexistence problems increase. On the other hand, with the increased need for ensuring application-specific minimum quality-of-service levels (e.g., in audio and video communication), the need for a clean and interference-free channel is desired more than ever. To date, non-collaborative techniques have been discussed to address these issues. In non-collaborative techniques, of which CSMA, DFS, and even spreading are examples, each system operates autonomously in order to minimize the impact of interference imposed by others. Collaborative techniques on the other hand, require systems to communicate with each other in order to find an optimal solution where mutual interference is minimal. Transceivers using the same standard can communicate with each other, but the same is not generally true when transceivers designed for operation in accordance with differing systems are considered.